The present invention relates to an automobile security system, and more particularly, to a key assembly for use in a vehicle ignition and lock unit.
Various types of security systems used in conjunction with the ignition circuit of a vehicle are known in the art. Many of these systems include anti-theft and/or anti-tampering mechanisms which are incorporated to deter the unauthorized use of vehicles. An electronic vehicle ignition lock is a component of some of these systems which can increase vehicle security and even lower insurance rates in some countries.
An electronic interlock system uses a coded activation signal which enables a vehicle operator to run a vehicle. Coded activation signals are generally read electronically within an ignition lock and subsequently sent to an electronic control module. The electronic control module controls engine operation and enables the vehicle to run only if the correct activation signal is received. Therefore, an electronic interlock system prevents a vehicle from running even if the ignition lock is bypassed or pulled. This system makes vehicle theft more difficult and time consuming.
The ignition keys employed for use with electronic interlock systems traditionally contain mechanical and/or electronic interlock codes. One such system incorporates a resistor pellet in the ignition key. The resistor pellet provides a resistor of a resistance such that when the ignition key is inserted into and rotated within a vehicle""s ignition cylinder unit, an electrical current is applied through the resistor. A decoding circuit determines if the resistance of the resistor pellet in the ignition key is within a predetermined resistance xe2x80x9cwindow.xe2x80x9d If the resistance provided by the resistor pellet in the ignition key is within the predetermined resistance range, the vehicle will run. To the contrary, if the resistance falls outside of the predetermined resistance window, the vehicle will not run. Examples of these interlock systems and associated keys are illustrated in U.S. Pat. Nos. 4,250,482, 5,083,362 and 5,156,032.
In another electronic interlock system, radio frequency identification (RFID) is used in the enabling or disabling of engine operation. An RFID interlock system consists of a reader which sends a signal to an antenna associated with a transponder that is mounted in a key. The transponder includes a transponder circuit, which can comprise an integrated circuit or discrete components, and a resonant circuit formed by a capacitor and an inductor or coil. The signal energizes the transponder, and the transponder responsively transmits a unique identification code back to the reader which decodes the identification code. If the signal transmitted by the transponder represents a valid identification code, the reader transmits this information to the vehicle""s electronic control module thereby enabling engine operation. However, if the signal is not a valid identification code, the reader causes the electronic control module to prevent engine operation. Typically, the antenna associated with the transponder produces a relatively high energy electromagnetic field which is coupled to the coil of the transponder and converted to a DC voltage which is used to power the electronic circuits of the transponder. The transponder transmits its unique identification code in the form of a low energy radio frequency signal that is received and decoded by the reader as described above.
Precisely because RFID electronic interlock systems are such effective security devices, it is critical that these systems work dependably in all the conditions a vehicle might encounter. The result of an RFID system failure is that a vehicle owner, or other person properly in possession of the keys for a given vehicle, is stranded and unable to bypass the interlock system to operate the vehicle. Electronic interlock systems are industry specified. Industry test configurations and requirements reflect conditions and circumstances which RFID interlock systems might actually encounter and, therefore, are a fairly reliable indicator of the dependability of the security systems.
Older RFID systems used transponder chips that are packaged in glass vials, often containing silicone, in an effort to protect the electronic components contained within the vials. Developments in the electronic industry have resulted in transponders that are overmolded with plastic and such transponders have gained wide acceptance in RFID interlock systems for vehicles. Recently, the inventors have discovered that harsh operating conditions affect the performance of RFID interlock systems in which the transponders are packaged in plastic material. However, RFID systems employing such transponders molded in key heads meet industry requirements, only when used with relatively expensive receivers.
One of the most popular configurations of transponders currently available is produced by Texas Instruments, Inc., as Texas Instruments, Inc. part number RI-TRP-W9WK. Another popular transponder configuration is that manufactured by Motorola, as Motorola part no. 05504-001. Both transponders include an overmold of a plastic material with electronic components located substantially within the overmold.
The inventors have found a problem involving a shift in inductance of the coil and the capacitance of the capacitor of the resonant circuit of the transponder which affects the operation of RFID security systems which occurs when the transponders are used in conjunction with prior key assembly designs. A shift in the inductance and capacitance of the transponder""s resonant circuit changes the resonant frequency of the transponder which can result in failure of the transponder to receive the interrogation signal being transmitted, or in the weakening of the strength of the signal sent to the receiver so that the receiver cannot detect signals transmitted by the transponder. The end result is that the vehicle engine cannot be started and/or run using the ignition or otherwise. The shift is believed to result from mechanical and/or thermal effects which produce forces upon the transponder, ultimately shifting the resonant frequency of the transponder. The greatest shift in resonant frequency, occurs in cold temperatures. For the Motorola and Texas Instruments, Inc. transponders, resonant frequency shift can be up to about 7 KHz, depending upon material in which the transponder is molded.
Decreasing, with the ultimate goal of completely eliminating, the amount of resonant frequency shift associated with the transponder eradicates the problem described herein above concerning RFID ignition lock systems. Testing has demonstrated that shifting in the resonance of the transponder is reduced when there is minimum contact between the transponder and any substantially rigid material that supports the transponder in the key assembly. Testing also has demonstrated that reducing the mass of the substantially rigid material that is located adjacent to the transponder in the key assembly reduces the shift. These favorable results are believed to be attributable to minimizing external forces applied to the transponder by limiting the force transmitting ability of the substantially rigid material structure adjacent to the transponder.
In accordance with the invention, both mechanical and thermal considerations are incorporated into improving the key assembly design to make RFID systems more dependable. Mechanical considerations are addressed in the structural design of the key assembly, and thermal considerations are addressed through the careful selection of construction materials and structural design, so that a delicate balance is achieved in the improved key design provided by the invention.
Achieving a balance between the mechanical and thermal considerations is critical because of their interrelated nature. Constructing the key head of a relatively hard material alleviates mechanical problems concerning key head deformation or failure from shear or torsional forces. However, the nature of harder plastic material is such that it tends to cause higher forces to be applied to the transponder under temperature excursions. Correspondingly, using a relatively soft plastic material to construct the key head tends to abate thermally-related problems because such material is less likely to produce high forces on adjacent components than does a harder material. However, the softer material is more prone to mechanical deformation by externally applied forces. This softer material can also adversely impact the structural integrity of the key head. The present invention attains a delicate equilibrium between minimization of the adverse effects of thermal expansion and contraction and mechanical stability.
The operating characteristics of transponders can be changed by forces caused by thermal expansion and contraction, and by impact force and compressive force occurring during manufacturing of the key assembly, especially during the molding processes. Two procedures have been found by the inventors to alleviate damage due to impact force applied to the transponder during the molding process. These procedures include the use of a specially gated, two-step molding process for producing the key assembly and the use of a novel frame and mounting structure for supporting the transponder.
The present invention addresses a number of concerns that affect the operation of the transponder. One concern is breakage or other damage to the transponder due to mechanical forces applied to the transponder during the molding processes. Another concern is damage that can result due to the heat that is applied to the transponder during the molding process. A further concern is damage that can be caused by the shrinking or contracting of the undermold and/or overmold material during cooling of the key assembly following the molding operation. Yet another concern is changes in operating conditions, including but not limited to changes in temperature, in the daily operation of a key assembly that includes a transponder.
More specifically, the inventors have invented a method for molding the key assembly for vehicle ignition locks equipped with RFID systems that substantially eliminates the problem of resonant frequency shift. In accordance with highly preferred embodiments of the present invention, the transponder is first surrounded by an undermold using injection molding techniques. The undermold comprises a relatively hard plastic material which surrounds and protects the transponder from certain outside forces along its weakest axes and holds the transponder in the proper location within the key. Voids can be formed in the undermold during the first stage of the process. Then, the key assembly is overmolded, providing an outer covering that encloses and protects the transponder. During overmolding of the key assembly, the voids formed in the undermold provide a space for the overmold material to fill, which further secures the top and bottom center portions of the overmold, thereby increasing the integrity of the overmold and ensuring that the overmold of the key assembly will not separate and disfigure the key.
Further in accordance with the invention, the injection molding process is conducted so as to minimize impact forces applied to the transponder during the molding process. Preferably, the undermold material is injected, in liquid form, through a gate that directs the material against a corner of the transponder causing the material to be split into two portions. Consequently, the liquid material that forms the undermold encircles the transponder as the material is being injected producing substantially even hydrostatic pressures. Encircling the transponder with the liquid material (which will eventually harden to form the undermold of the key assembly), substantially prevents the application of impact forces directly to the planar surfaces of the transponder, with an attendant reduction in the potential for damage to the transponder which could cause the device to fail.
In accordance with a feature of the invention, during the injection molding process, strategically located voids are formed in the undermold. These voids, which can be extremely small, eliminate pressure differentials which can otherwise develop between the adjacent portions of the mold. Eliminating the potential for a pressure differential prevents the transponder from shifting or cracking within the key assembly during overmolding.
Another benefit of molding the key assembly in two stages, namely first undermolding and subsequently overmolding, is that while the plastic is cooling following the injection molding process, overall heat and compressive force imposed on the transponder are substantially reduced. While the harder plastic material undergoes a greater degree of compression during cooling, the impact on the transponder is minimized because less material is used in forming the undermold.
Moreover, the component tending to cause a shift in the resonance of transponders for key assemblies used in RFID systems is substantially eliminated using the overmolding process provided by the invention. The softer plastic material which is used to form the overmold portion of the key head tends to abate thermal problems because the overmold material is less inclined to exert pressure and distort the transponder to the degree that harder material would. However, softer material is more prone to mechanical deformation by shear or torsional forces.
A further benefit of the present invention is the use of an injection molding device which supports and contains the transponder within the key assembly along its weakest axis to prevent cracking, fracturing, and other adverse effects, any of which can contribute to failure. The mold plates forming the molds that are used in molding the undermold and the overmold of the key assembly further serve to reduce, even prevent thermal excursion during manufacture of the key assembly because the mold plates function as heat sinks. The mold plates absorb auxiliary heat and thermal energy so that the affects of the heat upon the transponder in both the undermold and overmold processes are substantially reduced. Preferably, the mold plates comprise a relatively massive material with good heat transfer characteristics.
The improved process for manufacturing RFID systems incorporating known transponders, such as those produced by Texas Instruments, Inc. and Motorola, minimizes the impact and compressive force applied along the weak axis (or axes in the case of the Motorola device) of the transponder and, thus, reduces the chance that the transponder will fail.